1. Technical Field
The present invention relates to semiconductor devices and methods for manufacturing the same, and more particularly to a semiconductor device that can restrict the substrate floating effect even in a transistor having a short gate length and long gate width.
2. Discussion
FIG. 11(a) shows a plan view of one example of a conventional semiconductor device, and FIG. 11(b) shows a cross-sectional view taken along lines 11b—11b indicated in FIG. 11(a). The semiconductor device includes a transistor with a SOI (Silicon On Insulator) structure.
As shown in FIG. 11(b), the SOI substrate 104 is formed from a supporting substrate 101 composed of single crystal silicon (Si), an embedded oxide film (BOX layer) 102 formed on the supporting substrate 101, and a single crystal Si layer 103 that is formed on the embedded oxide film 102. The SOI substrate 104 can be manufactured by a variety of manufacturing methods, for example, it can be manufactured by a bonding method, a SIMOX (Separation by Implanted Oxygen) method.
An element isolation film 106 is formed in the single crystal Si layer 103. A gate oxide film 109 is formed on the surface of the single crystal Si layer 103, and a gate electrode 105 is formed on the gate oxide film 109. Sidewalls 110 are formed on side walls of the gate electrode 105, and impurity diffusion layers 111 with a low impurity concentration are formed in the single crystal Si layer 103 below the sidewalls. Diffusion layers 107 and 108 for source/drain regions are formed in the single crystal Si layer 103 adjacent to the low impurity concentration diffusion layers 111.
The single crystal Si layer 103 under the gate electrode 105 defines a body region 112. The body region 112 extends outwardly from one end of the gate electrode 105 as shown in FIG. 11(a). The gate electrode 105 and the diffusion layers 107 and 108 for the source/drain regions are connected to external elements through contact sections 113–115, respectively. Also, the body region 112 is connected to a body terminal (not shown) through a contact section 116.
By the application of a specified voltage to the body region through the body terminal, the body potential is fixed and thus the substrate floating effect is controlled. The body potential is fixed because the state of the body potential is always unstable in operating the transistor unless the body potential is fixed, which substantially affects the transistor characteristics.
The conventional semiconductor device explained above is formed with a short gate length and a long gate width as shown in FIG. 11(a). In the semiconductor device having such a configuration, the body potential in an area that is far from the contact section 116 for fixing the body potential cannot be completely removed, and the substrate floating effect may occur. In other word, in a semiconductor device having a short gate length and a long gate width, the resistance in the body region is high. As a result, the body potential cannot be sufficiently fixed, and the substrate floating effect may occur.
Also, in the conventional semiconductor device described above, the body region extends out from one end of the gate electrode in order to form a contact section therein for fixing the body potential. However, as the body region is extended in this manner, the area occupied by the element increases, which is problematical because it hinders miniaturization of elements.
The present invention has been made in view of the circumstances described above, and its object is to provide a semiconductor device that can restrict the substrate floating effect even in a transistor having a short gate length and long gate width, and a method for manufacturing the same.